1. Field of the Invention
The present invention relates to a local area network (LAN) system which comprises a plurality of terminals and achieves communication among the terminals, and more particularly to an ATM-LAN system which operates in asynchronous transfer mode (ATM) to accomplish communication among terminals.
2. Description of the Related Art
ATM (Asynchronous Transfer Mode) communication has been attracting attention as promising technique of increasing the speed of transfer/exchange operation. In the ATM communication, information is transferred from one terminal to another, in the form of fixed-length short packets generally known as "cells." The ATM communication is a technique of exchanging the packets by means of hardware in order to facilitate high-speed transfer/exchange of information.
ATM communication is considered to be "a promised solution" to the broadband aspects of an integrated services digital network (B-ISDN) which will be employed in the future. In recent years, active research and development have been conducted on so-called ATM-LANs which covers a limited region, for example, one floor, and to which ATM technique is applied to enhance the inter-operability so that the ATM-LANs may be incorporated into a larger communication network. However, the ATM-LANs thus far developed have the following problems.
Basically, an ATM communication network provides connection-orientated communication by means of an ATM layer. A conventional ATM-LAN has a so-called network side function which sets any connection to terminals connected to the ATM-LAN. The technique utilizing frequently networked services provided on the LAN, such as a distribution operating system, i.e., the technique of "when a certain terminal receives a specific service, searching for a host computer providing the specific service by means of intra-LAN broadcast" must be achieved by utilizing a system which is provided for performing a network side function. Each terminal has but a low independence with respect to the network side function. This results in a great problem that the network side function limits the function of services which are to be networked.
Known as a LAN in which the terminals are greatly independent of the network side function is Ethernet (trademark of Xerox). In Ethernet, as well known in the art, a plurality of terminals are connected to one another by a coaxial cable. Ethernet is assumed to utilize only the signal-transferring function of the coaxial cable as a one network side function. In Ethernet, the terminals are connected by the coaxial cables in accordance with a protocol which is generally known as "CSMA/CD." As widely known, in CSMA/CD, each terminal does not transmit a message via the coaxial cable before it determines that no other terminal is transmitting a message. If two or more terminals simultaneously started transmitting messages, the messages would collide with one another in the coaxial cable. Should they collide, the messages could no longer be transferred correctly. Thus, in Ethernet, each terminal determines whether or not messages are colliding in the coaxial cable, before it transmit a message through the coaxial cable. Collision of messages takes place with a certain probability. With Ethernet it is difficult to allocate the bandwidth on the coaxial cable to each terminal. This means that Ethernet cannot transfer data with a guaranteed QOS (Quality Of Service) among the terminals as is required in multimedia application.
Among LANs wherein bandwidth can be allocated to a certain terminal are a token ring network and a token path network. In both types of networks, the terminals exchange among themselves an access right called "token." The bandwidth on the coaxial cable can be allocated to each terminal, by changing the method for allocating the bandwidth to the terminal which has obtained the token. If errors are made in exchanging the token among the terminals, all terminals of the LAN can no longer communicate with one anther. To prevent this, the LAN should be provided a protocol for monitoring the exchange of the token and would inevitably be complex and expensive.
As has been indicated, the conventional ATM-LAN is disadvantageous in that each terminal is insufficiently independent of the network. On the other hand, the conventional ATM-LAN wherein each terminal is greatly independent of the network is disadvantageous in that the bandwidth cannot be allocated to the terminals, or in that it is complex and, hence, expensive, in order to enable the terminals to exchange tokens among themselves.